Olefins, including ethylene and propylene, are converted into a multitude of intermediate and end products on a large scale, mainly into polymeric materials. Commercial production of olefins is mostly accomplished by the thermal cracking of hydrocarbons. Unfortunately, due to the very high temperatures involved, these commercial olefin producing processes also yield a substantial amount of the less desired acetylenic (alkyne) impurities such as acetylene, methylacetylene and C4 alkynes which contaminate the target olefin streams and therefore need to be removed. The separation of the acetylenes from the olefins can considerably increase the plant cost. Propylene, for example, can contain several percent methylacetylene and propadiene (collectively referred to as “MAPD”). A selective hydrogenation (SH) reaction with hydrogen in presence of supported metal catalysts is the most common method for MAPD removal. In spite of significant progress over the years, this process has significant shortcomings such as the need for a source of hydrogen, the appearance of side products such as green oil and propane, and catalyst deactivation from impurities such as arsine or carbonyl sulfide.
Several methods are known for separation of an organic gas containing unsaturated linkages from gaseous mixtures. These include, for instance, cryogenic distillation, liquid adsorption, membrane separation and pressure swing adsorption in which adsorption occurs at a higher pressure than the pressure at which the adsorbent is regenerated. Cryogenic distillation and liquid adsorption are common techniques for separation of carbon monoxide and alkenes from gaseous mixtures containing molecules of similar size, e.g. nitrogen or methane. However, both techniques have disadvantages such as high capital cost and high operating expenses. For example, liquid adsorption techniques suffer from solvent loss and need a complex solvent make-up and recovery system.
Beside palladium and modified palladium, copper with some additives can be used also as a catalyst for selective hydrogenation as seen in U.S. Pat. No. 3,912,789 and U.S. Pat. No. 4,440,956. Kokai JP Number 50929-1968 describes a method of purifying vinyl compounds containing up to about 10 percent by weight of acetylenic compounds. In this method, acetylenic compounds were described as being adsorbed on an adsorption agent of 1-valent and/or 0-valent copper and/or silver supported on inert carrier such as Δ alumina, silica or active carbon. Separations described included 1000 ppm ethyl acetylene and 1000 ppm vinyl acetylene from liquid 1,3-butadiene, 100 ppm acetylene from ethylene gas, 100 ppm methylacetylene from propylene gas, and 50 ppm phenyl acetylene from liquid styrene (vinylbenzene). Each application used fresh adsorption agent and only a short time of one hour on stream at mild conditions of temperature and pressure. Such limited applications were likely because it is well known that acetylene and these acetylene compounds react with copper and/or silver to form copper acetylide or silver acetylide. Both the acetylide of copper and silver are unstable compounds. Because they are explosive under some conditions, their possible formation presents safety problems in operation and in handling adsorbent containing such precipitates. A current commercial process employs a copper based catalyst in the presence of hydrogen.
It is known that acetylenic impurities can be selectively hydrogenated and thereby removed from such product streams by passing the product stream over an acetylene hydrogenation catalyst in the presence of dihydrogen (molecular hydrogen, H2). However, these hydrogenation processes typically result in the deposition of carbonaceous residues or “green oil” on the catalyst which deactivates the catalyst. Therefore, acetylene hydrogenation processes for treating liquid or liquefiable olefins and diolefins typically include an oxygenation step or a “burn” step to remove the deactivating carbonaceous residues from the catalyst, followed by a hydrogen reduction step to reactivate the hydrogenation catalyst. For example, see U.S. Pat. No. 3,755,488 to Johnson et al., U.S. Pat. No. 3,792,981 to Hettick et al., U.S. Pat. No. 3,812,057 to Morgan and U.S. Pat. No. 4,425,255 to Toyoda. However, U.S. Pat. No. 3,912,789 and U.S. Pat. No. 5,332,705 state that by using selected hydrogenation catalysts containing palladium, at least partial regeneration can be accomplished using a hydrogenation step alone at high temperatures of 316° to 371° C. (600° to 700° F.) and in the absence of an oxygenation step.
Selective hydrogenation of the about 2000 to 4000 parts per million (ppm) of acetylenic impurities to ethylene is generally a crucial operation for purification of olefins produced by thermal steam cracking. Typical of a small class of commercially useful catalysts are materials containing very low levels of an active metal supported on an inert carrier, for example a particulate bed having less than about 0.03% (300 ppm) palladium supported on the surface skin of carrier pellets having surface area of less than about 10 m2/g.
Many commercial olefin plants using steam crackers use front-end acetylene converters, i.e. the hydrogenation unit is fed C3 and lighter cracked gas, which has a high enough concentration of hydrogen to easily hydrogenate the acetylenic impurities; however, when run improperly, will also hydrogenate a large fraction of the ethylene and propylene product. Both hydrogenation of acetylene and ethylene are highly exothermic.
One technology used for removal of acetylenes is described in U.S. Pat. No. 6,124,517 assigned to BP Amoco. This patent discloses the removal of acetylenes from olefin streams by adsorption in absence of hydrogen over a copper—alumina adsorbent containing Cu in a reduced, zero covalent state. Hydrogen containing gas is then used to regenerate the adsorbent.
The present invention provides an efficient method for purification of hydrocarbon streams, olefins in particular, by removing acetylenes from the hydrocarbon stream in the absence of hydrogen. The acetylenes are converted to a diacetylene form which can be easily separated from the hydrocarbon stream due to the high boiling and melting points of the diacetylenes. One of the possible applications of the present invention is as a pre-treatment in combination with a selective hydrogenation step in which the load on the selective hydrogenation unit is significantly reduced and allowing for the processing of a significantly greater sized hydrocarbon stream.